A common application for robotics is to position an object without changing its orientation. In some cases “Cartesian” robots with multiple serial prismatic joints are used. However, prismatic joints present significant design challenges. Unlike revolute joints, which can use compact, precise, and low-cost anti-friction rotational bearings, linear guides are larger, heavier, more expensive, and more difficult to maintain. For example, linear guide surfaces must maintain their surface properties and geometry over the entire length of motion. These surfaces are also susceptible to wear, dirt, and moisture, and are difficult to cover and protect.
In practice, serial-chain robots with revolute joints are more commonly used for these tasks. However, these revolute joint robots require extra joints to keep the object's orientation from changing. For example, to translate an object in two dimensions without changing its orientation, only two prismatic joints are required. For the same task, three revolute joints are required, increasing the cost and complexity of the device.
For precision positioning of objects over small distances, such as micromanipulators, four-bar linkages with leaf-spring flexures are often used instead. When restricted to motion ranges that are small relative to the lengths of the leaf-spring flexure elements, these devices produce near-linear motion. At higher motion ranges, however, the motion deviates significantly from linear, and the leaf-spring forces increase proportionally, making them less effective.
Actuation for these devices is also complicated for larger motion ranges. Typically some form of linear actuator is used to push a drive surface connected to the output portion of the device. At large motion ranges, these linear actuators are no longer aligned with the motion direction, reducing efficiency. The linear actuator tip must also slide along the drive surface an increasingly large amount as the motion range increases, resulting in increased wear, parasitic friction, and side loading of the actuator, none of which is desirable in precision applications.
U.S. Pat. No. 5,587,937 describes a four-bar linkage with cable drive actuation to drive an adjacent link. This configuration has the advantage of keeping the motor bulk and mass on the distal end of the device, but does not allow the bulk and mass to be located in the middle of the linkage, which is preferable for a modular actuator. In addition, this configuration applies forces to one end of the linkage while interaction forces are applied at the opposite end, increasing moment loading and necessitating stiffer bearings and linkages.